1. Field of the Invention
The present invention relates a fuel-cells system, particularly to a fuel-cells system comprising a fuel cell which receives a supply of gaseous fuel and a supply of oxidizing gas, and generates an electromotive force.
2. Description of the Prior Art
Regarding a fuel cell that produces electromotive force by electrochemical reactions when supplied on the anode side with a gaseous fuel containing at least hydrogen and on the cathode side with an oxidizing gas containing at least oxygen, it is well known from Nernst's equation set out below that increasing the hydrogen partial pressure of the gaseous fuel or the oxygen partial pressure of the oxidizing gas elevates the electromotive force E.sup.T of the cell and also improves the power generating efficiency. EQU E.sup.T =E.sup.O +(RT.multidot.ln(P.sub.H2 .multidot.P.sub.O2.sup.1/2 /P.sub.H2O))/2F (1)
E.sup.T : Open voltage PA0 E.sup.O : Theoretical voltage calculated from free energy PA0 F: Faraday constant PA0 R: Gas constant PA0 T: Temperature PA0 P.sub.H2 : Hydrogen partial pressure PA0 P.sub.O2 : Oxygen partial pressure PA0 P.sub.H2O : Water partial pressure PA0 P.sub.O2 : Oxygen partial pressure of oxidizing gas supplied to cathode side PA0 X.sub.O2 : Oxygen concentration of the oxidizing gas supplied to the cathode side PA0 P: Total pressure of oxidizing gas supplied to cathode side
In the oxidizing gas supplied to the anode side, the oxygen partial pressure of the oxidizing gas is related to the oxygen concentration and the supplied gas total pressure as follows: EQU P.sub.O2 =P.multidot.X.sub.O2 /100 (2)
It follows from Equation (2) that the oxygen partial pressure of the oxidizing gas can be increased by increasing the total pressure of the oxidizing gas. A widely adopted conventional practice has therefore been to incorporate a compressor into the fuel-cells system and to supply the fuel cells with air compressed by the compressor as oxidizing gas. The aim of this practice is to raise the total pressure of the air (i.e., the oxidizing gas) so as to increase the oxygen partial pressure of the oxidizing gas and thus increase the electromotive force of the fuel cells. It also follows from Equation (2) that higher oxygen partial pressure of the oxidizing gas can also be achieved by raising the oxygen concentration of the supplied oxidizing gas, meaning that power generating performance of the fuel cells can be further enhanced by increasing the oxygen concentration.
Methods proposed for raising the oxygen concentration of the oxidizing gas supplied to a fuel cell include that of supplying the fuel cell with an oxidizing gas obtained by removing nitrogen from air by use of a nitrogen separator installed in the fuel-cells apparatus, as taught by Japanese Patent Laid-open Gazette No. 6-140067, for example. Specific methods known for supplying a fuel cell with oxygen-enriched air obtained by separating nitrogen from air include the method of supplying a fuel cell with an oxidizing gas obtained by enriching the oxygen content of air by the PSA (Pressure Swing Absorption) method (taught, for example, by Japanese Patent Laid-open Gazette No. 4-190570) and the method of supplying a fuel cell with oxygen selectively separated from air by use of an oxygen permeable membrane (taught, for example, by Japanese Patent Laid-open Gazette No. 3-276576). By use of these methods, the oxygen concentration of the oxidizing gas supplied to the fuel cell can be increased to enhance the power generating performance of the fuel cell.
The PSA method consists in passing air removed of water vapor and carbon dioxide through a molecular sieve made of zeolite or the like to cause mainly nitrogen to be adsorbed by the zeolite and thereby obtain an oxidizing gas of high oxygen partial pressure. The PSA method can provide an oxidizing gas having very high oxygen partial pressure (oxygen concentration of 90% or more). The PSA method also has disadvantages, however. One is that the large amount of zeolite or other adsorbent needed increases the size of the apparatus. Another is that a large amount of electric power is needed to drive the device for effecting the method. These drawbacks make the PSA method difficult to apply particularly in the case where the fuel cells of a fuel-cells system installed in an electric vehicle are used as a power source for driving the vehicle.
In the method of separating oxygen from air by use of an oxygen permeable membrane, the permeable membrane constantly receives a pressure falling within a prescribed range during oxygen separation. The low durability of the permeable membrane therefore makes the device for effecting this method incapable of stable oxygen separation over a long period. To secure an adequate oxidizing gas flow by this method, moreover, the amount of air processed has to be increased by using a large area permeable membrane and/or making the pressure differential between the opposite sides of the permeable membrane great. However, the size of the overall device increases in proportion to the area of the membrane and energy consumption rises in proportion to the pressure differential. The method using an oxygen permeable membrane is therefore difficult to adopt for supply of oxidizing gas to fuel cells for powering an electric vehicle.